Are you not changing your version of BES and as long as your database is external of the bes server. simply build a bes server with the same SRP key. Do not connect them to the network at the same time. Shut one server down unplug the network cable. Power the new server up plug in the network connection. All the bes users connect to the new server with no problems.
Then decommision your old bes. and migrate your exchange environment.
It is that easy. But you will probably go with adding a new building a new bes server new encyption key use the same database. adn move users from one server to another.
I perfer metod one the big bang.